1. Field of the Invention
The invention relates to electroerosion printers and particularly to a print head for metal paper printers with feedable or wear-compensating electrodes flexibly resting on the recording medium.
2. Description of the Prior Art
From German Pat. No. 1,266,030 (U.S. Pat. No. 3,419,886) a print head for metal paper printers is known, whereby the print electrodes are feedably mounted in the print head, flexibly resting under a slight pressure on the recording medium.
The adjusting means described comprise a sheet-metal disk around which an electrode wire is wound, one end of which is clamped in the disk. With one electrode end touching the recording medium, the wire section remote from the disk rests at a particular bending stress against the inside walls of a contact frame, thus ensuring that the electrode wire flexibly rests on the recording medium. If the print electrodes wear on their free end, the electrode wire initially deflects and can be readjusted at particular intervals by manually turning the disk.
As a result of the dimensions of the electrodes used, this arrangement permits only low resolutions. In accordance with the above-mentioned patent, the electrodes are designed as leaf springs. This provides a practicable solution in the case of thicker electrodes. For higher resolutions, i.e. when using electrodes with a small diameter, this solution is impracticable because of the insufficient mechanical strength of the electrodes and their relatively high wear.
It is generally known that electroerosive recording methods for metallized paper as a recording medium require permanent, i.e. uninterrogated, contact between the electrodes and the metal layer. This means that the electrodes are permanently forced to slide across the paper. This sliding and the erosion process connected with the evaporation of parts of the metal layer during printing cause the electrodes to wear. This wear is the greater, the thinner the electrodes. For a low resolution, thicker electrodes (which are mechanically stable) can protrude from the print head body to such an extent that the print head's life suffices for many applications.
Thinner electrodes on the other hand, as are required for higher resolutions, are mechanically unstable and subject to higher wear. Therefore, they have to be embedded nearly up to their printing tip in a non-conductive supporting body of the print head. Total embedding right up to the tip is undesirable, since in such a case permanent contact between electrodes and recording medium would not be ensured as a result of the natural roughness of the latter. (See also FIG. 2.) If the electrodes of the print heads are permitted to protrude from the latter by an amount roughly equalling the electrode strength (to ensure an adequate buckling strength), the life of such print heads is insufficient, since the electrodes wear too rapidly. Even if the material (e.g. plastics) used to embed the electrodes is softer and more highly wearing than the material (molybdenum, tungsten) of which the electrodes are made, additional means have to be employed to ensure that the plastics material wears to such an extent that the electrodes invariably protrude from the print head body by an adequate amount. Therefore, it is necessary to refeed the electrodes according to their wear. This solution has already been employed for low-resolution print heads, i.e. in connection with thick electrodes. For thin electrodes, i.e. high-resolution print heads, such a requirement becomes even more pressing in view of the higher wear.